1. Field of the Invention
The present invention relates to a power source and a power failure detection apparatus provided on an image forming apparatus such as a copying machine, a printer, and a facsimile machine.
2. Description of the Related Art
A state where an alternating current (AC) voltage is not supplied from a commercial AC power source to an apparatus (which is also referred to as “power failure state”) includes a case where the supply of an AC voltage from the commercial AC power source is cut off due to power failure, a case where the supplied AC voltage falls below the specifications of the apparatus, and a case where a power cable is pulled out by a user while the apparatus is operating. It is desirable to stop a power source circuit after the power failure state is detected and the apparatus is shifted to a state where the apparatus can be normally stopped. Japanese Patent Publication No. 4080764 discusses a method for shifting to a state where the apparatus can normally stopped such that a first converter (an insulative DC/DC converter) and a second converter (an insulative DC/DC converter) are connected in parallel to a full-wave rectification circuit of the power source, thereby detecting the power failure state and stopping the second converter to cause the first converter to continue supplying power for a certain period of time.
There has been known a fixing device which is provided on an image forming apparatus such as a copying machine, a printer, and a facsimile machine, fixes an image to a recording medium, and includes an endless belt, a ceramic heater coming into contact with the inner surface of the endless belt, and a pressure roller forming a nip portion with the ceramic heater via the endless belt. As a means of controlling a power supplied to the fixing device, a method is used which performs phase control of the power supplied from the commercial AC power source using a switching element such as a triac. The phase control of the supplied AC-voltage waveform requires accurately detecting a timing when the AC voltage becomes zero volts, i.e., zero cross timing (hereinafter referred to “zero cross”) as timing being the reference of phase control. Japanese Patent Application Laid-Open No. 2003-199343 discusses a circuit for detecting the zero cross timing.
In a case where the power failure state is detected by detecting the AC voltage when a power cable is pulled out by the user, among the above power failure state, a decrease in voltage of the power source circuit is delayed by the influence of electric charges charged in an X capacitor provided for inhibiting noises of the power source, taking time to detect the power failure state. The resistance of a resistor discharging the X capacitor is reduced to allow the delay to be improved. On the other hand, a further reduction in power in a standby state in which the apparatus is not operated is strongly demanded, so that the power consumption of a circuit for detecting the power failure state also needs to be reduced or inhibited. In other words, an early detection of the power failure state and a reduction of power consumption in the standby state of the apparatus are demanded.
In recent days, there has been a demand for further reduction in power consumption during a state where the apparatus is not operated (an operation standby state). If an X capacitor discharging resistance for discharging the X capacitor is provided, the X capacitor discharging resistance consumes power with the apparatus being not operated. In other words, the X capacitor discharging resistance impedes reduction in power consumption in the operation standby state. The power consumption of the power source circuit is increased, so that a method is used in which a state where a power cable is pulled out is detected and the X capacitor is discharged. On the other hand, Japanese Patent Publication No. 4446136 discusses a configuration in which a state where a power cable is pulled out is detected to discharge the X capacitor, as a method for discharging the X capacitor while the power consumption of the power source is being reduced. In the method discussed in Japanese Patent Publication No. 4080764, little discharge current flows into the X capacitor in a state where the apparatus is not operated (an operation standby state), so that power consumption can be reduced.
The method discussed in Japanese Patent Publication No. 4446136 has a problem that the circuit scale of the power source increases. Therefore, there has been demanded a reduction in power consumption with a simple circuit configuration.